This invention relates to an oil separator which is attached to the outlet side of a compressor such as the type used in a comparatively small refrigerator, for example, for an automobile air conditioner to thereby separate lubricant from gaseous refrigerant. The gaseous refrigerant (hereinafter called as gas), which is exhausted from the compressor of an automobile air conditioner, is moderately loaded with particles of lubricant which is intermixed with the gas in the compressing operation so that if this gas is directly supplied to a condenser, this gas exerts a bad influence upon heat transfer in the heat transmission surface of the cooling pipes and thus cooling is extremely impeded.